Neutron radiation detectors are used in sensing neutrons emanating from a radiation source. Typically, neutrons are only emitted from a very limited set of man-made materials. This makes neutron detectors useful in detecting the presence of such made-made sources of neutron radiation, such as are typically present within radiological materials.
A general example of a Helium-3 (3 He) neutron detector is shown in FIG. 1. The detector includes a metal tube filled with 3He gas under high pressure, such as 3 or more atmospheres. Centrally located within the metal tube is a wire electrode which is biased by an external high voltage supply coupled through a resistor (R). The high voltage is typically is on the order of +1200 V with respect to the metal tube housing of the detector which is held at ground potential. The signal generated by the detector when neutrons interact with the 3He gas is coupled by a capacitor (C) to a charge sensitive preamplifier that provides an amplified voltage pulse to downstream electronic circuits that count the number of neutron interactions that have occurred within the detector.
The voltage pulse at the output of the preamplifier typically has a rise time less than 2 microseconds and a fall time constant (also referred to as tail recovery time) on the order of 10 or more microseconds. The rise time is usually dictated by the bandwidth and slew rate of the preamplifier components, the capacitance of the interconnections between the detector and the preamplifier, and the nuclear collection characteristics of the detector. The tail pulse decay time constant is determined by an RC time constant within the preamplifier (not shown in FIG. 1).
The voltage pulses at the output of the preamplifier are further processed to enable counting of the neutron interaction events, and in some instances to measure the pulse amplitudes. The typical use of this type of detector is in counting the number of events larger than some noise threshold, thereby allowing measurement of the neutron flux in counts per second.
When a detector like that shown in FIG. 1 is subjected to vibration, the internal wire electrode will move with respect to the outer metal casing. This movement of the internal electrode, which is at the high electrical potential, with respect to the outer metal casing, which is at the ground electrical potential, injects charge into the preamplifier. This injected charge, referred to herein as microphonic noise, is amplified by the preamplifier in the same fashion as normal signals from the detector. The microphonic noise will appear as voltage waveforms at the preamplifier output. The presence of such noise can cause the output of the preamplifier to saturate and/or increase the difficulty in accurately detecting the real neutron interaction events in the subsequent processing stages.
Prior neutron detection systems have attempted to amplify the detector signal as high as possible and then process the preamplifier waveform using a simple comparator. This approach causes the preamplifier or subsequent amplifier stages to saturate, thereby rendering them “blind” to subsequent pulses from the detector. If the microphonic noise is not of sufficient amplitude to totally saturate the preamplifier or subsequent amplifiers, it can still cause the comparator which follows to fire repeatedly as the microphonic noise crosses the set threshold. This causes bursts of false neutron counts as the microphonic noise continues. In some prior systems, attempts have been made to detect either of these conditions and turn off the reporting of any events when these conditions occur. Other systems have tried to detect the microphonic vibrations with accelerometers or other means to initiate algorithms to blank out the response of the systems to the microphonic noise.
It is conceivable that these weaknesses in prior detector systems could be purposefully exploited to allow illicit neutron-emitting sources to pass by undetected. In this respect, microphonic noise is analogous to a “jamming” signal in a communication or radar application. Preventing these radiation detectors from being “jammed” is one of the purposes of the present invention.
What is needed, therefore, is a system for removing or minimizing microphonic noise from a radiation detector signal without creating excessive false counts in the electronics that count the radiation events.